In a conventional two shaft gas turbine, a gas generator unit and a power turbine unit are mounted together at its axial ends along an axial direction of the gas turbine.
FIG. 8 shows a conventional two-shaft gas turbine 200. The gas turbine 200 is separated into a gas generator unit 201 and a power turbine unit 202.
In the compressor section of the gas generator unit 201, mechanical work of rotor blades is converted into fluid energy of a working fluid streaming through the gas turbine 200. The temperature of the working fluid is then further raised in the combustion section of the gas generator unit 201. Inside a turbine section of the gas generator unit 201, the working fluid expands and drives a shaft of the gas generator unit 201 connecting the turbine and the compressor of said unit. In the turbine section of the gas generator unit 201, fluid energy of the working fluid is converted to mechanical work by driving of the rotor blades. Approximately 50% (percent) of the energy of the working fluid converted to mechanical work is used for driving the blades the compressor section of the gas generator unit 201.
The working fluid flows further from the gas generator unit 201 to the power turbine unit 202. In a turbine section of the power turbine unit 202, the energy of the working fluid is converted in mechanical work for generating e.g. power for external use. Thereby the working fluid drives a further shaft of the power turbine unit 202, which further shaft is generally decoupled from the shaft of the gas generator unit 201.
The gas generator unit 201 and the power turbine unit 202 comprise functional units, such a gas inlet unit, compressor unit, turbine unit, which are arranged adjacent to each other along an axial direction of the gas turbine. Each functional unit is installed in an individual casing. Hence, the gas generator unit 201 comprises an inlet casing 203, a compressor casing 204, a center casing 205 including the combustion section and a compressor turbine casing 206. The power turbine unit 202 comprises e.g. a power turbine casing 207 and an exhaust casing 208.
Generally, the gas turbine 200 is supported during operation by a front beam 212 and two rear struts 211. The front beam 212 and the rear struts 211 support the gas turbine 200 to a ground 220. The front beam 212 supports, e.g. the inlet casing 203 of the gas generator unit 201. The inlet casing 203 is pivotably mounted to the front beam 212, wherein the pivoting axis of the inlet casing 203 extends generally parallel to an axial direction of the gas turbine 200. A further functional unit, such as the power turbine casing 207, is pivotably supported by the two rear struts 211, wherein the power turbine casing 207 is pivotable with respect to the rear struts 211 around a pivoting axis which is perpendicular to the pivoting axis of the front beam 212.
In order to remove the gas generator unit 201, e.g. for maintenance reasons, the functional unit which is closest to the power turbine unit 202 (i.e. the compressor turbine casing 206) has to be split off from the adjacent functional unit of the power turbine unit 202, i.e. the power turbine casing 207. In order to remove the gas generator unit 201, the gas generator unit 201 is connected to a sliding rail system on the ground that allows axial and transverse movement of the gas generator unit 201 with respect to the power turbine unit 202. For supporting the gas generator unit 201 independently after the gas generator unit 201 is split off from the power turbine unit 202, e.g. two temporary front struts have to be fixed to the inlet casing 203 and a temporary center strut has to be fixed to e.g. the compressor turbine casing 206, so that the gas generator unit 201 is provided with a three point support (the temporary center strut and the two temporary front strut assemblies). These temporary struts are connected to the sliding rails of the sliding rail system, so that the gas generator unit 201 can be slid forward away from the power turbine unit 202.
The temporary front support strut assemblies are not flexibly mounted to the casing. Hence, the front support strut assemblies are assembled to the inlet casing 203 during the maintenance of the gas generator unit 201 but have to be disconnected during the running of the gas turbine 200 to allow underbase twist (ground twist) to take place without stressing the casings.
During operation of the gas turbine 200, the rear support is provided by the horizontal pins of the rear struts 211 and by the front pin of the front beam 212, so that the ground twist is isolated from the engine casings. If the temporary support strut assemblies were to be attached during operation, the twist of the ground 212 would be transferred by the struts to the gas turbine casing (e.g. the inlet casing 203) which would harm the inlet casing 203, for example. In particular, there is a potential failure mode where the temporary support strut assemblies are left in place during engine running which could lead to casing damage or more seriously to rotor or bearing damage. A more serious failure mode would be if the temporary front support struts were not fitted prior to separation of the gas generator from the power turbine as then there would only be an unstable two point support for the gas generator. This failure mode does not apply to the temporary central strut 209, since this has to be in place in order to be able to separate the gas generator 201 from the power turbine 202.
The above-described method for splitting the gas generator unit 201 from the power turbine unit 202 is complex and time-consuming, e.g. because the temporary struts have to be assembled and disassembled for each maintenance cycle.
There is also a potentially lethal failure mode where the struts are not fitted prior to engine disassembly, leading to toppling of the gas generator on disconnection from the power turbine.
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